LUIS F. LELOIR 



followed by disappointment when sucrose phosphorylase could 

 not be detected in plant tissues. 



More recently, another enzyme has been found (17) which 

 catalyzes the formation of sucrose as follows : 



UDPG + fructose , UDP + sucrose (21) 



In this case the enzyme was found to be present in the seeds 

 of several plants, and the equilibrium constant is about 5 (AF 

 about -1000). 



Reaction (21) was believed to explain the long-sought 

 mechanism of sucrose synthesis in plants, until another enzyme 

 was found which catalyzes the following reaction : 



UDPG + fructose-6-phosphate , ^ UDP + sucrose phosphate 



(22) 



Wheat germ was found to contain both enzymes, and since 

 phosphatase is also present, the reaction product may be rapidly 

 dephosphorylated to free sucrose and accumulates only when a 

 purified enzyme is used. 



A sucrose phosphate was detected by Buchanan, Calvin, and 

 others (3) as a product of photosynthesis in algae and green 

 leaves. It is not known whether it is the same as that formed in 

 reaction (22), which has the phosphate group in position 6 of the 

 fructose moiety. 



A study of the relative activities of the enzymes which syn- 

 thesize sucrose and sucrose phosphate in different parts of the 

 plant would be of physiological interest, but has not been carried 

 out owing to technical difficulties. Very likely sucrose phos- 

 phate follows some metabolic path different from that of free 

 sucrose, so that it is apt to become the subject of interesting in- 

 vestigations. 



Another disaccharide has been synthesized enzymically start- 

 ing with UDPG. This is trehalose phosphate. Yeast extracts 

 have been obtained (15) which catalyze the reaction: 



UDPG + glucose-6-phosphate > trehalose phosphate + UDP 



(23) 



604 



